Potentiometric cell with dual slope display meter calibration circuit

ABSTRACT

A system for performing analysis of the ion concentration of test solutions including means for sensing the potential developed between a reference electrode and an ion selective electrode immersed in the test solution, and means for amplifying and displaying the potential. The amplifier and display means include two independent span adjustment controls, each effective over part of the span of the meter. Automatic stepless transfer is provided between the two span adjustment controls, for example at mid-scale. This enables increased accuracy when displaying ion concentrations by permitting a dual slope straight line approximation to the typically nonlinear millivolt characteristic of the ion selective electrode such as occurs in the presence of interfering ions.

United States Patent Pratt Nov. 6, 1973 POTENTIOMETRIC CELL WITH DUALSLOPE DISPLAY METER CALIBRATION CIRCUIT [75] Inventor: Stanley L. Pratt,Riverside, Calif.

[73] Assignee: Beckman Instruments, Inc.,

Fullerton, Calif.

[22] Filed: May 4, 1972 [21] Appl. No.: 250,286

[52] 11.8. CI..... 204/195 M, 204/195 R, 204/195 G,

[51] Int. Cl. G01n 27/46 [58] Field of Search 204/1 T, 195 G, 195 L,204/195 R, 195 M [56] References Cited UNITED STATES PATENTS 3,591,4647/1971 Frant et al 204/195 M Primary Examiner-T. Tung Att0rneyJames M.Thomson et al.

[57] ABSTRACT A system for performing analysis of the ion concentra tionof test solutions including means for sensing the potential developedbetween a reference electrode and an ionselective electrode immersed inthe test solution, and means for amplifying and displaying thepotential. The amplifier and display means include two independent spanadjustment controls, each effective over part of the span of the meter.Automatic stepless transfer is provided between the two span adjustmentcontrols, for example at mid-scale. This enables increased accuracy whendisplaying ion concentrations by permitting .a dual slope straight lineapproximation to the typically nonlinear millivolt characteristic of theion selective electrode such as occurs in the presence of interferingions.

3 Claims, 1 Drawing Figure PATENTEU NOV 6 I973 SHUT 2 CF 2 NNQPOTENTIOMETRIC CELL WllTll-I DUAL SLOPE DISPLAY METER CALIBRATIONCIRCUIT BACKGROUND OF THE INVENTION This invention relates to a systemfor determining the concentration of a selected ion within a testsolution. More particularly the invention concerns a system for sensing,amplifying and accurately displaying the potential developed between anion selective sensing electrode and a reference electrode immersed inthe test solution.

The analysis of test solutions by monitoring electrochemical reactionsis well known, and in particular the use of potentiometric cells indetermining ion concentration is known. Such cells, as commonly used formonitoring ion concentrations, include a cell confining a test solutionin which a reference electrode and one or more ion selective sensingelectrodes are immersed. The concentration of the ion of interest isdetermined by measuring and displaying the potential developed betweenthe reference electrode and a given ion selective sensing electrode.

In the design of a meter suitable for displaying the concentration of aparticular ion, it is appropriate to use a logarithmic scale, since themillivolt output of the sensing electrode is generally proportional tothe logarithm of the electrochemical activity of the particular ionpresent. Thus, it is well known that an ideal electrode would exhibitNernstian response, i.e.,'at 25C an ideal electrode would always producea 59.16 millivolt change per ten fold change of concentration of the ionbeing sensed. Under such ideal conditions the potential or millivoltoutput of the electrode would be linear over several decades of changeof ion concentration.

Unfortunately, the change in millivolt output of a given electrodedepends not only upon the concentration of the primary ion of interest,but also upon its relative ionic strength and upon the presence andnature of other interfering ions. For these reasons it is often foundthat equal logarithmic increments of concentration of the primary ion donot result in identical millivolt changes. A display meter having asingle continuously adjustable span or slope control and a continuouslyadjustable asymmetry or offset 'control can be calibrated to such asignal at only two points on the meter display. If such a meter is used,then for display of an ion characteristic having more than one decade ofconcentration change, significant errors in the output displayed on themeter will result.

If on the other hand a meter were to be used which 'pro-vided twoindependent span adjusting controls, each effective over part of themeter span then three point calibration of the meter display would bepossi ble, with correspondingly decreased errors.

SUMMARY OF THE INVENTION Accordingly it is an object of the invention toprovide an improved system for determining and displaying theconcentration of an ion within a test solution.

It is another object of the invention to provide in such a system, adisplay meter that includes two independent span adjustment controlseach effective over part of the span of the meter.

It is a further object of the invention to provide in a meter havingsuch span adjustment controls, means for automatic stepless transferbetween the two span adjustment controls, for example at mid-scale,whereby an ion concentration display of increased accuracy is provided.

The above and other objects of the invention are attained in a systemwhich includes means for confining a test solution with a referenceelectrode and at least one ion selective sensing electrode immersedtherein. The potential developed between the reference electrode and agiven sensing electrode is amplified and provided as the input signal toa differential amplifier having a common input resistor and twoalternately operable feedback networks. For a negative input to theresistor a first diode connects one of the feedback networks through adownscale span potentiometer, while for a positive input another diodeconnects the other feedback resistor through an upscale scanpotentiometer. By use of the alternate feedback networks, a polarityseparator circuit is provided having two outputs, one for positiveoutputs from the amplifier and the other for negative outputs from theamplifier. Either positive or negative 'input voltages to the amplifierare continuously and linearly adjustable by the downscale and upscalespan potentiometers for full deflection from the meter center point.Accordingly this arrangement allows different ion selective electrodesto be connected and standardized by a three point procedure bycalibrating one point at meter zero'and one additional point at eachside of meter zero.

BRIEF DESCRIPTION OF THE DRAWINGS The foregoing and other objects of theinvention are apparent from the following detailed description of theinvention taken in conjunction with the accompanying drawing which is aschematic diagram of one preferred embodiment of the invention.

DETAILED DESCRIPTION OF THE INVENTION Referring now to the drawing, onepreferred embodiment of the invention is illustrated comprising a system10 for sensing, amplifying and accurately displaying ion concentrationsof a test solution. The system includes a cell 12 of conventional designadapted to hold a sample of the test solution within which a referenceelectrode l4 and two ion selective sensing electrodes 16, 18 areimmersed. The reference electrode can comprise a standard calomelreference electrode of well-known design. The ion selective electrodesare also of conventional design and may comprise glass electrodes suchas those disclosed in US. Pat. Nos. 3,278,399 and 3,143,448. If desired,the sensing electrodes can be of other conventional type.

Reference electrode 14 is connected by a conductor 21 to the movable arm24 of a switch 25. Switch 25 further includes fixed contacts 27, 28which are adapted for selective connection with movable arm 24. Contact27 is connected by a conductor 31 to the movable arm 32 of apotentiometer 34. One end terminal 35 of the potentiometer is groundedand the other end terminal 36 of the potentiometer is connected to asuitable d.c.

source, not illustrated, which provides energization for the electrodesin a well-known manner when movable arm 24 is associated with contact27.

Contact 28 of switch 25 is connected via a conductor 41 to a movable arm42 of a potentiometer 44. One end 45 of the potentiometer is groundedand the other end 46 is adapted to be connected to a dc. source, againnot illustrated, to provide a different level of energization for thereference electrode when movable arm 24 is associated with contact 28.The movable arm of either of the potentiometers can be adjusted to varythe energization of reference electrode 14 whereby a continuouslyadjustable asymmetry or offset control is provided for the system, aswill become apparent hereinafter.

Electrode 18 is connected by a conductor 51 to a fixed contact 58 of aswitch 55. In similar fashion, electrode 16 is connected by a conductor52 to a fixed contact 57 of switch 55. A movable arm 54 of switch 55 isconnected by a conductor 59 to a positive input terminal 61 of a highinput impedance differential amplifier 60. Output terminal 63 ofamplifier 60 is connected to one end of a resistor 66. The other end ofresistor 66 is connected by a feedback conductor 67 to the negativeinput terminal 62 of the amplifier. The other end of resistor 66 is alsoconnected to one end of a resistor 68 having its other end grounded. Inthe configuration shown, amplifier 60 is a conventional high inputimpedance differential amplifier connected in a noninverting mode with avoltage gain fixed by the ratio of resistances 66 and 68.

Output terminal 63 of amplifier 60 is connected by a conductor 71 to afixed contact 91 of a polarity reversing switch 93. In similar fashionoutput terminal 63 is connected by conductors 71, 73 to a fixed contact101 of a polarity reversing switch 103. The output terminal is alsoconnected via conductors 71, 75 to one end of an input resistor 76having its other end connected to the negative input 81 of adifferential amplifier 80. Positive input terminal 82 of the amplifieris connected to ground. An output terminal 83 of amplifier 80 isconnected to one end of a feedback resistor 86 having its other endconnected to negative input terminal 81. Differential amplifier 80 is ofconventional design and is connected, in the configuration shown, as aunity gain inverting amplifier.

Output terminal 83 of amplifier 80 is connected by a conductor 87 to afixed contact 92 of switch 93 and.

terminal 83 is also connected by a conductor 88 to a fixed contact 102of switch 103.

A movable arm 90 of switch 93 is connected by a conductor 105 to a fixedcontact 112 of a switch 115, and a movable arm 100 of switch 103 isconnected by conductor 106 to a fixed contact 113 of switch 115. A

movable arm 1 14 of switch 1 15 is connected to one end of an inputresistor 117 having its other end connected to the negative inputterminal 121 of a differential amplifier 120. The positive inputterminal 122 of the amplifier is connected to ground.

An output terminal 123 of amplifier 120 is connected to the anode of adiode 125 and to the cathode of a diode 126. The cathode of diode 125 isconnected by a conductor 131 to one terminal of a capacitor 134 whichhas its other terminal connected by a conductor 132 to negative inputterminal 121 of the amplifier. In similar fashion, the anode of diode126 is connected by a conductor 141 to one terminal of a capacitor 144having its other terminal connected by a conductor 142 to negative inputterminal 121 of the amplifier.

A downscale span potentiometer 155 and an upscale span potentiometer 156are provided each having one end connected to a grounded terminal 157.The other end of potentiometer 155 is connected to a first outputterminal 165 and the other end of potentiometer 156 is connected to asecond output terminal 166.

A movable arm 154 of potentiometer is connected by a conductor 151 toone terminal of a feedback resistor 152 having its other terminalconnected to conductor 132 and ultimately to negative input terminal 121of the amplifier. In similar fashion, a movable arm 153 of potentiometer156 is connected by a conductor 171 to one terminal of a feedbackresistor 172 having its other terminal connected to conductor 142 andultimately to negative input terminal 121 of the amplifier. Outputterminal is connected via a conductor 173 and conductor 131 to thecathode of diode 125. In similar fashion output terminal 166 isconnected via a conductor 174 and conductor 141 to the anode of diode126.

Amplifier 120 is a conventional differential amplifier which, in theconfiguration shown, is connected as an inverting polarity separator.Accordingly for negative inputs to the amplifier the gain is determinedby feedback resistor 152 via span potentiometer 155. However forpositive inputs to the amplifier the gain is determined by feedbackresistor 172 via span potentiometer 156. Since both negative feedbackcircuits have high forward gain, the diode forward voltage drops arenegligibly small when referred to the amplifier input. Accordingly anypolarity switching discontinuities near zero input voltage areinsignificant. This enables stepless transfer between thepotentiometers. Feedback capacitors 134, 144 limit the upper frequencybandwidth and suppress a.c. signals, such as those of input powerfrequency, that might cause the span controls not to be independent nearmeter zero values.

Output terminal 165 is connected by a conductor 181 to one terminal of asumming resistor 182 and output terminal 166 is connected by a conductor185 to one terminal of a summing resistor 187. The other terminals ofsumming resistors 182, 187 are each connected to a common terminal 188.

Terminal 188 is connected by a conductor 189 to a negative inputterminal 191 of a differential amplifier 190. The positive inputterminal 192 of amplifier is connected to ground. An output terminal 193of amplifier 190 is connected by a conductor 194 to the positiveterminal of a meter 195. The other terminal of meter 195 is connected bya conductor 197 to one terminal of a feedback resistor 198 having itsother terminal connected by a conductor 199 to negative input terminal191 of the amplifier. The other terminal of meter 195 is also connectedvia conductor 197 and conductor 201. to one terminal of a resistor 202having its other terminal connected to ground.

Amplifier 190 is a conventional differential amplifier which, in theconfiguration shown, acts as a summing amplifier for the output of thepolarity separating amplifier. Feedback resistor 198 determines thevoltage gain of the amplifier and resistor 202 determines the resultingcurrent through meter 195.

As shown, the movable arms of switches 93, 103 are connected togetherfor gang operation. In similar fashion, the movable arms of switches 25,55 and 115 are connected together for gang operation.

In operation, the circuit illustrated can be utilized for displaying,selectively, the potential between reference electrode 14 and either ofsensing electrodes 16, 18. Furthermore, the sensing electrodes can be ofeither cation or anion type. The electrode selected for display isdetermined by ganged function switches 25, 55, 115 with switch 55selecting the electrode and switch 25 selecting the asymmetry or offsetpotentiometer to be utilized with that electrode. Switch 115 connectsthe input of the polarity selector circuit to the corresponding polarityswitch 93, 103. The polarity switches are then moved to the appropriateposition to display either positive (cation) or negative (anion)signals.

When the appropriate electrode, standard solution and polarity switcheshave been selected, the appropriate offset potentiometer 34, 44 isemployed to set the mid-scale position of the meter. In the preferredembodiment the mid-scale position is at zero, although it can be set atanother suitable value if desired. After mid-scale standardization,standard solutions are employed in the septum and the span adjustpotentiometers 155, 156 are utilized to standardize the meter at twoadditional points, one point on either side of meter zero. Consequentlythe meter output can be calibrated to display ion concentration directlywith higher accuracy than was heretofore possible with two pointstandardized meters. This enables satisfactory use of the ionconcentration sensing system to measure concentrations of a primary ionin the presence of secondary or interfering ions.

With such an arrangement the meter span need not represent exactly twodecades of concentration change, nor must it be logarithmic. However thecircuit arrangement does provide increased accuracy when displaying ionconcentration by allowing a dual slope straight line approximation tothe typically non-linear millivolt characteristics of ion selectiveelectrodes in the presence of interfering ions.

I claim:

1. An ion concentration sensing and display system comprising ionselective sensing means having ion selective electrode means adapted forimmersion in a test solution for monitoring a potential related to theion concentration of a selected ion within the solution, said sensingmeans including adjustable offset potentiometer means for adjusting theoffset of said potential with respect to a given ion concentration;polarity separating amplifier means connected to said sensing means forproducing outputs of positive and negative polarity, said polarityseparating amplifier means including means for individually adjusting.the amplifier gain with respect to each of I said outputs; and displaymeans connected to said polarity separating amplifier means for directlydisplaying the concentration of the selected ion.

2. The ion concentration sensing and display system of claim 1 furtherincluding polarity inverting amplifier means and polarity switchingmeans for selectively interposing said invert-ing amplifier meansbetween said sensing means and said polarity separating amplifier meanswhereby the ion concentration of anions or cations can be directlydisplayed.

3. The ion concentration sensing and display system of claim 2 whereinsaid display means includes summing amplifier means connected to saidpolarity separating amplifier means and a display meter connected to theoutput of said summing amplifier for directly displaying ionconcentration.

1. An ion concentration sensing and display system comprising ionselective sensing means having ion selective electrode means adapted forimmersion in a test solution for monitoring a potential related to theion concentration of a selected ion within the solution, said sensingmeans including adjustable offset potentiometer means for adjusting theoffset of said potential with respect to a given ion concentration;polarity separating amplifier means connected to said sensing means forproducing outputs of positive and negative polarity, said polarityseparating amplifier means including means for individually adjustingthe amplifier gain with respect to each of said outputs; and displaymeans connected to said polarity separating amplifier means for directlydisplaying the concentration of the selected ion.
 2. The ionconcentration sensing and display system of claim 1 further includingpolarity inverting amplifier means and polarity switching means forselectively interposing said invert-ing amplifier means between saidsensing means and said polarity separating amplifier means whereby theion concentration of anions or cations can be directly displayed.
 3. Theion concentration sensing and display system of claim 2 wherein saiddisplay means includes summing amplifier means connected to saidpolarity separating amplifier means and a display meter connected to theoutput of said summing amplifier for directly displaying ionconcentration.